set default python to python3
set default python to python3

Set the default python to python3 except for the py27 environment. We
have to set that explicitly to override the new default.

Change-Id: Ica596235831a51cb02afb46c5e1a167a32cf6bf5
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/oslo.service